The invention concerns an impeller of a so-called closed type for pumps, compressors, fans, etc. of a centrifugal or semiaxial type.
Impellers of this type are characterized by being arranged to rotate within a housing into which a liquid or a gas is fed through a central axial opening. The medium flows through one or several channels, where the energy is increased, and is finally expelled at the periphery of the impeller. The velocity of the medium is normally decelerated, thus giving the medium an additional static pressure increase.
An impeller of a so-called closed type comprises a cover disc (shroud) having a central hole for medium coming into the impeller, another cover disc (hub) heading the driving unit and a number of vanes arranged between the cover discs, which vanes are curved and which between themselves form channels for transport of the medium towards the periphery. There may be different numbers of vanes and designs depending on the type of medium that is transported, the volume rate, the head, etc.
When pumping liquids containing solid bodies, such as waste water, etc., it is desirable to have as large a free passage as possible through the impeller. It is therefore common to design the impeller to have one single vane as this allows the largest possible free passage. An example of such an impeller is shown in the Swedish Patent No. 7903729-7. One disadvantage with this type of impeller, in addition to manufacturing problems, is that it is not symmetrical and therefore difficult to balance. Another disadvantage is that the efficiency is not as high as possible. It is therefore common to use impellers with several vanes, in spite of their more narrow passages An example of such an impeller is shown in the Swedish Patent No. 306 706.
In German Patent Application No. 35 30 985 there is shown how it is possible to decrease the flow losses in an impeller by designing the cover discs rotationally non-symmetrical. A reduction of the distance between the cover discs on the suction side is proposed, however, this will not have any significant influence on the secondary flow, since the divergence in the meridian plane is mainly the same on the suction and the pressure sides of the impeller. This arrangement thus does not provide any significant reduction of the loss that is due to secondary flow.